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A crab megalopa larva, magnification x 40. This larval stage is short-lived, and invdividuals feed voraciously on other zooplankton. After about a week, the megalopa sinks to the sea bed where it molts into a juvenile crab. Photo: Richard R. Kirby

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Beneath the ocean’s surface lies a thriving alien world—one that’s largely invisible to us because its inhabitants are so tiny. The vast, sunlit reaches under the waves are home to innumerable plankton in all their bizarre varieties. With hues ranging from translucent to shockingly bright yellows and reds, these creatures resemble graceful spirals, glittering gems, spiked globes, winged snails, fierce crabs, and even spaceships.

Using a microscope, Richard Kirby offers an up-close look at these strange and beautiful animals in his new book, Ocean Drifters. “Plankton include everything from the tiniest of free-living bacteria and viruses up to animals as large as jellyfish,” he says. “Between those extremes, you have this huge diversity of life.” Unlike fish, which can swim against the flow of water, all plankton are essentially at the mercy of ocean currents. Their name, in fact, is derived from the Greek word planktos, or “drifter.” They may be microscopic, says Kirby, a molecular geneticist at Plymouth University in the United Kingdom. “But they’re incredibly important to life on earth—not just life in the sea but the concentration of gases in our atmosphere, the geology of the land around us, and the petrol that people burn in their cars.”

In the ocean, plankton underpin the entire food web. There are two broad types: phytoplankton—plantlike cells—and zooplankton, which eat phytoplankton or other zooplankton. In addition to their key role as a food source, phytoplankton account for about half of photosynthesis on the planet, making them a principal oxygen producer. “People typically think of the rainforest as lungs of the earth, but for every second breath you take, the oxygen comes from these tiny organisms no bigger than the diameter of a strand of human hair,” Kirby says. Grazers of the sea, herbivorous zooplankton feed on phytoplankton. These vegetarians range from single-celled protozoa to multicellular shrimplike creatures, and are, in turn, eaten by predatory zooplankton such as crab larvae, which are gobbled up by fish. Some of the world’s largest fish, including the 40-foot-long basking shark, feed directly on zooplankton.

Ocean biodiversity is thought to rival that of the tropics, with scientists uncertain just how many species each might hold. For his part, Kirby identifies plankton by looking at their DNA sequences. The work is particularly urgent as sea temperatures rise due to climate change. “As the sea surface warms, the plankton that live there are shifting their distributions and changing in their abundance,” Kirby says. “If we don’t understand the diversity of the plankton, we can’t really understand the full extent of the changes that are occurring—the ramifications for the commercial fisheries, for example, or for organisms higher up the food web that rely upon the plankton.”

These tiny creatures also influence the climate. Phytoplankton act as a carbon sink, consuming carbon dioxide that dissolves in the ocean from the atmosphere. Longer term, over millions of years, their dead remains become incorporated in sedimentary rock (when heated and compressed, some become fossil fuels). Ocean Drifters contains portraits of more than 70 species of plankton, each accompanied by a short caption packed with fascinating tidbits. “I wanted the images to captivate people,” says Kirby. “And, by engaging them with the beauty of these organisms, to help them understand the diversity of life and how the components fit together to create the world as we know it.”